Broadband optical retardation device

ABSTRACT

A broadband optical retardation device, such as may be used for polarisation encoding of display information or in diffractive optical systems, includes a patterned uniform half wave plate retarder in combination with a non-patterned uniform quarter wave plate retarder having an optic axis oriented at 90° to the xz plane. The patterned retarder consists of alternating first and second regions having first and second optic axes at different orientations to a reference axis, for example at +22.5° and −22.5° to the xz plane. Considering light of wavelength λ° incident on the retarder and linearly polarised in the xy plane, such light is differently polarised by the regions, and the light output by the device includes regions in which the light is right circularly polarised and regions in which the light is left circularly polarised. In the case of incident light of wavelengths greater or less than λ°, the output light becomes progressively more linearly polarised as the wavelengths departs to a greater extent from the ideal value, but not to the same extent as would be the case if the device consisted simply of a patterned single layer acting as a quarter plate retarder.

TECHNICAL FIELD

This invention relates to broadband optical retardation devices, and isconcerned more particularly with the use in such devices of patternedoptical retarders in which the optic axis varies over the retarder inaccordance with a predefined pattern.

BACKGROUND ART

By a “broadband optical retardation device” is meant a retardationdevice for light consisting of a broad range (of the order of tens orhundreds of nanometers) of wavelengths which constitutes at least a partof the complete wavelength range of visible light and ultraviolet andinfrared radiation.

Such optical retardation devices may be used for polarisation encodingof display information or in diffractive optical systems, for example.

S. Pancharatnam “Achromatic Combination of Birefringent Plates”,Proceedings of Indian Academy of Sciences vol XLI, No 4, Sec A, 1955, pp130-136 and pp 137-144, discuss the use of stacked uniform retarderfilms to improve achromaticity, that is the independence of its lighttransmission properties from the wavelength of the input light. Use ismade of combinations of uniform retarder films having differentazimuthal orientations of their optic axes. A method for calculating therequired retardations and orientations of the optic axes is also given.However these references are concerned only with combinations of uniformretarders, that is retarders whose optic axis is substantially invariantover the retarder.

It is known to fabricate patterned retarders, that is retarders whoseoptic axis vary over the retarder, for example between first and secondregions which alternate in x and/or y directions over the plane of alayer. European Published Patent Application No. 0689084 (Schadt)proposes the use of reactive mesogen layers as optical elements andalignment surfaces.

It is an object of the invention to provide an optical retardationdevice having improved achromaticity which is capable of generatingbroad band orthogonal optical modes.

DISCLOSURE OF INVENTION

According to the present invention there is provided a broadband opticalretardation device for receiving light consisting of a broad range ofwavelengths, the device including patterned optical retardation meansincluding a first region having a first optic axis at an orientation abetween 0° and +90° to a reference plane and a second region having asecond optic axis at an orientation b between 0° and −90° to thereference plane, and non-patterned optical retardation means having anoptic axis at a defined orientation c, greater than the orientation aand less than the orientation 180°+b, to the reference plane in order toincrease the achromaticity of the light polarised by the combination ofthe patterned optical retardation means and the non-patterned opticalretardation means as compared with the light polarised by the patternedoptical retardation means alone.

It should be understood that, in the above definition and elsewhere inthe specification, the term “optic axis” is used to denote the so-calledslow optic axis of the material referred to.

Such a combination of patterned optical retardation means andnon-patterned optical retardation means in this manner enablesgeneration of broad band orthogonal optical modes in a straight forwardmanner, and permits a broad bandwidth response to be obtained withimprovement in the quality and/or ease of fabrication as compared withknown broad band optical retardation devices.

The first and second regions of the patterned optical retardation meansare preferably such as to polarise input light linearly polarised alongthe reference plane such that, after passing through the non-patternedoptical retardation means, the light which has passed through the firstregion is orthogonal to the light which has passed through the secondregion. It will be understood that the required orthogonal relationshipbetween the polarised light from the first regions and the polarisedlight from the second regions can be satisfied whether the light islinearly polarised or circularly polarised.

In a preferred embodiment of the invention the orientations a and b ofthe first and second axes of the patterned optical retardation means aresubstantially equal and opposite relative to the reference plane, andthe orientation c of the optic axis of the non-patterned opticalretardation means is substantially perpendicular to the reference plane.Such an arrangement optimises the broad band response of the device.

The orientations a and b of the first and second axes of the patternedoptical retardation means are preferably in the ranges of +10° to +75°and −10° to −75° respectively, and most preferably in the ranges of +10°to +35° and −10° to −35°, relative to the reference plane. The optimumresponse is obtained if the first and second orientations a and b of thefirst and second axis of the patterned optical retardation means areabout +22.5° and −22.5° respectively relative to the reference plane.

In one embodiment of the invention the orientations a and b of the firstand second optic axes of the patterned optical retardation means and theorientation c of the optic axis of the non-patterned optical retardationmeans substantially satisfy the relationships c=a+45° and b=c−45°.

The patterned optical retardation means may include a patterned uniformlayer having an optic axis which varies between the first and secondregions along one or more directions x and y parallel to the layer, butwhich does not vary substantially through the thickness of the layer.

Alternatively the patterned optical retardation means may include apatterned twisted retardation layer having an optic axis which variesbetween the first and second regions along one or more directions x andy parallel to the layer and also through the thickness of the layer.

Furthermore the non-patterned optical retardation means may include auniform retardation layer whose optic axis has an orientation which doesnot vary substantially through the thickness of the layer.

Alternatively the non-patterned optical retardation means may include atwisted retardation layer whose optic axis has an orientation whichvaries through the thickness of the layer.

The invention also provides an optical retardation device includingpatterned optical retardation means including a first region having afirst optic axis configuration and a second region having a second opticaxis configuration, and non-patterned optical retardation means having afurther optic axis configuration, at least one of the optic axisconfigurations being a twisted optic axis configuration whose optic axishas an orientation which varies through the thickness of a layer,whereby the achromaticity of the light polarised by the combination ofthe patterned optical retardation means and the non-patterned opticalretardation means is increased as compared with light polarised by thepatterned optical retardation means alone.

In one embodiment of the invention the first optic axis configurationhas an average optic axis orientation a between 0° and +90° to areference plane, the second optic axis configuration has an averageoptic axis orientation b between 0° and −90° to the reference plane, andthe further optic axis configuration has an average optic axisorientation c, greater than the orientation a and less than theorientation 180°+b, to the reference plane.

In an alternative embodiment of the invention the first optic axisconfiguration has an output director orientation a between 0° and +90°to a reference plane, the second optic axis configuration has an outputdirector orientation b between 0° and −90° to the reference plane, andthe further optic axis configuration has an output director orientationc, greater than the orientation a and less than the orientation 180°+b,to the reference plane.

In order that the invention may be fully understood, reference will nowbe made, by way of example, to the accompanying drawings in which:

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of a patterned uniform retarder;

FIG. 2A is a schematic diagram of a broadband optical retardation devicein accordance with the invention consisting of a combination of apatterned uniform half wave plate and a non-patterned uniform quarterwave plate;

FIG. 2B is a table showing the ellipticity of light outputted by such adevice as a function of wavelength;

FIG. 3 is a schematic diagram of a further broadband optical retardationdevice in accordance with the invention consisting of a combination of apatterned uniform half wave plate and a non-patterned uniform half waveplate;

FIG. 4A is a schematic diagram of a further broadband opticalretardation device in accordance with the invention consisting of acombination of a patterned twisted half wave plate and a non-patterneduniform half wave plate;

FIG. 4B is a table showing the ellipticity of light outputted by such adevice as a function of wavelength;

FIG. 5 is a graph showing variation of the orientation of the optic axisover the two wave plates of the device of FIG. 4;

FIG. 6A is a schematic diagram of a further broadband opticalretardation device in accordance with the invention;

FIG. 6B is a graph of the transmission level against wavelength of lightreceived in use of an embodiment of the invention;

FIG. 7A schematically shows successive operational steps a to e in apossible fabrication method;

FIG. 7B schematically shows a liquid crystal display (LCD) incorporatinga broadband optical retardation device in accordance with the invention;and

FIG. 8 schematically shows a diffractive optical system incorporating abroadband optical retardation device in accordance with the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 schematically shows a patterned optical retarder 1 having apatterned layer 2 applied to a substrate (not shown) and consisting ofalternating first and second regions 3 and 4 having first and secondoptic axes 5 and 6 at different orientations to a reference axis 7. Moreparticularly the optic axis 5 is orientated at an orientation a between0° and +90° to the reference axis 7, and the optic axis 6 is orientatedat an orientation b between 0° and −90° to the reference axis 7, theangles a and b preferably being equal and opposite.

Whilst FIG. 1 shows the particular case of the patterned retarder 1consisting of alternating squares, and second regions 3 and 4, forexample alternating bands, having differently orientated optic axes, itshould be appreciated that patterned retarders may also be used whichconsist of regions of more than two types, that is three or more regionshaving optic axes of different orientations arranged in regularsequences or irregularly. For example, three uniform half wave platesarranged at −22.5°, +22.5° and 0° to a reference axis may be followed bya uniform quarter wave plate arranged at 90° to the reference axis.Incident linear light polarised along the 0° direction can then bepatterned into right circular, left circular and linear light. If twodetectors (information channels) sensitive to right and left polarisedlight follow this patterned optical element, then the right and leftcircularly polarised information only passes down one channel, whereasthe linearly polarised information is shared substantially equallybetween the two channels. The two detectors may, for example, be theleft and right eye of a viewer of a stereoscopic panel, where the lefteye is covered by one circular polariser and the right eye is covered bythe orthogonal circular polariser. Information that is linearly encodedrepresents those parts of an image that could be viewed by both eyes.Information that is circularly encoded represents those parts of animage that could be viewed by one eye only.

Generally, if the layer 2 is considered as being positioned parallel toa plane (x, y) where x and y are orthogonal directions and thez-direction is normal to the plane of the layer 2, the unit vector whichpoints along the local optic axis within the layer 2 may be termed thedirector n(x, y, z). In the case of an in-plane retarder n_(x)−0°.Furthermore in the case of a uniform retarder (whether patterned or not)n_(x) and n_(y) do not vary as a function of z, whereas in the case of atwisted retarder (whether patterned or not) n_(x) and n_(y) vary asfunction of z, that is n_(x)=n_(x)(z) and n_(y)=n_(y)(z). Where theretarder is patterned, so that the orientation of the optic axis variesacross the layer, n_(x) and n_(y) vary as a function of x and/or y, thatis n_(x)=n_(x) (x, y) and n_(y)=n_(y) (x, y), whilst not substantiallyvarying as a function of z. However, in the case of a patterned twistedretarder, n_(x) and n_(y) vary as a function of z and well as a functionof x and/or y, that is n_(x)=n_(x) (x, y, z) and n_(y)=n_(y) (x, y, z).

Thus, in the case of the patterned uniform retarder 1 shown in FIG. 1,the direction of the projection of the optic axis n=(n_(x), n_(y),n_(z)) onto the plane (x, y) varies as a function of position n=n (x, y)but the magnitude of the optical retardation does not vary as a functionof position (x, y). In the particular case shown the layer 2 consists oftwo regions 3 and 4 of retardance Δnd−λ₀/2 where Δn represents thechange in refractive index across the layer, d represents the thicknessof the layer and λ₀ represents the mean wavelength of incident light.The layer 2 therefore represents a half wave plate. Furthermore theoptic axes 5 and 6 are preferably at +22.5° and −22.5° to the xz plane(a=+22.5°, b=−22.5°). Particular embodiments of the invention utilisingsuch a patterned half wave plate retarder 1 will now be described, byway of example, with the reference to FIGS. 2A and 3, like referencenumerals being used in those figures to denote similar parts.

FIG. 2A shows such a patterned uniform half wave plate retarder 1 incombination with a non-patterned uniform quarter wave plate retarder 10formed by a layer 11 parallel to the layer 2 having a retardanceΔnd=λ₀/4 and an optic axis 12 having an orientation at 90° to the xzplane, that is perpendicular to the average of the optic axes 5 and 6 ofthe two regions 3 and 4 of the patterned layer 2. In the case of theretarder 10 the orientation of the optic axis 12 is substantiallyunvarying over the layer 11.

Considering light of wavelength λ₀ which is incident on the layer 4 inthe direction z and is linearly polarised in the xy plane as shown at14, such light is differently polarised by the regions 3 and 4 of thelayer having optic axis 5 and 6 at +22.5° and −22.5° to the xz plane,and the light output by the device includes regions in which the lightis right circularly polarised and regions in which the light is leftcircularly polarised. In the case of incident light of wavelengthsgreater or less than λ₀ as the wavelength departs to a greater extentfrom the ideal value, the output light becomes progressively morelinearly polarised, but not to the same extent as would be case if thedevice consisted simply of a patterned single layer acting as a quarterwave plate retarder. Thus the combination of the patterned uniform halfwave plate retarder 1 and the non-patterned uniform quarter wave plateretarder 10 serves as a broad band patterned quarter wave plate retarderin which the polarisation of the two components of the output light isless sensitive to variations in the wavelength of the incident lightthan is the case with conventional patterned quarter wave plateretarders.

This is demonstrated by the table of FIG. 2B which shows the calculatedellipticity of the output light for the two cases of (a) conventionpatterned single layer retarder designed to act as a quarter wave platefor incident light at a wavelength of 580 nm, and (b) the combination ofa patterned half wave plate retarder 1 and a non-patterned quarter waveplate retarder 10 as shown in FIG. 2A, the two layers 2 and 11 of such acombination again being designed for incident light having a wavelengthof 580 nm. This table clearly demonstrates the more achromatic circularpolarisation by use of the combination of FIG. 2A as the wavelength isvaried above and below the design wavelength of 580 nm. The table doesnot allow for wavelength dispersion of the refractive indices.

FIG. 3 shows an alternative embodiment in which a patterned uniform halfwave plate retarder 1 is used in combination with a non-patterneduniform half wave plate retarder 15 consisting of a non-patterned layer16 having a retardance Δnd−λ₀/2 and an optic axis 17 at 90° to the xzplane. As in the previously described embodiment the optic axes 5 and 6of the layer 2 are respectively at +22.5° and −22.5° to the xz plane.Light 14 of wavelength λ₀ linearly polarised along the xy plane andincident on the layer 2 in the z direction produces light at the outputside of the device consisting of two components linearly polarised at+45° and −45° to the xz plane. In this case the output light becomesprogressively less linearly polarised as the wavelength of the incidentlight is varied either side of λ₀, but not to the same extent as wouldbe the case in use of a conventional patterned single layer half waveplate retarder. Thus, whilst the embodiment of FIG. 3 is designed toproduce linearly polarised light components, whereas the embodiment ofFIG. 2A is designed to produce circularly polarised light components,the embodiment of FIG. 3 provides similar broad band behaviour to theembodiment of FIG. 2A.

In both the embodiments described above with reference to FIGS. 2A and3, the patterned and non-patterned layers are both uniform, that is theorientation of the optic axes of the patterned and non-patterned layersdo not vary through the thickness of the layers. However, as alreadymentioned, it is also possible to make use of patterned and/ornon-patterned twisted layers, that is layers in which the orientation ofthe optic axis varies through the thickness of the layer. FIG. 4A showsan embodiment of the invention in which a patterned twisted half waveplate retarder 20 is used in combination with a non-patterned uniformquarter wave plate retarder 10 (similar to that described with referenceto FIG. 2A). The patterned twisted retarder 20 consists of a layer 21including first and second regions 22 and 23 having optic axes havingorientations which vary through the thickness of the layer 21. In theparticular example given the orientations of the optic axes vary frombeing parallel to the xz plane at the input side of the layer, as shownat 26 and 27, to being at orientations of +37° and −37° respectivelyrelative to the xz plane at the output side of the layer 21, as shown at28 and 29. There is also shown in FIG. 4A graphs of the orientations Oof the optic axes of the retarders 10 and 20 against position in the zdirection through the thickness of the retarder, a and b being theorientations of the optic axes of the regions 22 and 23 of the retarder20 and c being the orientation of the optic axis 12 of the retarder 10.

FIG. 4B is a table showing the ellipticity of the output light of such acombination as a function of the wavelength of the incident light, undersimilar conditions to those used for the table of FIG. 2B. This shows afurther improvement in the achromatic circularly polarisation ascompared with the conventional single quarter wave plate retarder forwhich corresponding ellipticity values are shown in FIG. 2B.

FIG. 5 is a further graph showing the variation in the orientations aand b of the regions 22 and 23 of the layer 21, as shown in FIG. 4A, asa function of position along the z direction, as well as the constantorientation c of the optic axis 12 of the layer 11. It will beappreciated from this graph that the orientation a of the optic axis ofthe first region 22 varies linearly with position from 0° to +37° fromthe input side to the output side of the layer 21, whereas theorientation b of the optic axis of the second region 23 varies linearlywith position from 0° to −37° from the input side to the output side ofthe layer 21, giving in both cases an optical retardation of 1186.1 nm.Furthermore the optical retardation of the layer 11 having its opticaxis 12 at a uniform orientation c of 90° across the layer provides anoptical retardation of 106.7 nm. Clearly incident light having apolarisation axis at 0° will be converted to right or left handedcircular polarised light depending on whether it is transmitted throughthe region 22 or the region 23 of the layer 21.

FIG. 6A diagrammatically shows a further embodiment of the invention inwhich a patterned uniform half wave plate retarder 1 is used incombination with a non-patterned uniform half wave plate retarder 15, asin the embodiment of FIG. 3, except that in this case light 14 linearlypolarised along the xy plane and incident on the layer 2 in the zdirection produces light at the output side of the device consisting oftwo components linearly polarised at 0° and 90° to the xz plane. This isachieved by the optic axes 5 and 6 of the layer 2 having orientations aand b and the optic axis 17 of the layer 16 having an orientation cwhere these orientations satisfy the relationships c=a+45° and b=c−90°,where a=+22.5°, b=−22.5° and c=+67.5°. Light transmitted through theregion 4 of the layer 2 and then through the layer 16 remains linearlypolarised at 0° for all wavelengths, whilst light passing through theregion 3 of the layer 2 and then through the layer 16 becomes linearlypolarised at 90° at the wavelength for which the device is designed andclose to this orientation for other wavelengths. Such an embodiment, foruse as a parallax barrier for instance, is disclosed in British PatentApplication No. 9804500.8.

Although not specifically described with reference to the drawings, itwill be appreciated that it is also possible to provide an embodiment ofthe invention in which a patterned uniform retarder is combined with anon-patterned twisted retarder. Furthermore other embodiments may beprovided in accordance with the invention in which the optic axes of thetwo regions of the patterned retarder are not symmetrically orientatedwith respect to the reference axis and/or in which more than two regionsof different optic axis orientation are provided. Where only two suchregions of different orientation are provided, it is generally arequirement that the average optic axis (or alternatively the outputdirector) of the non-patterned retarder is orientated along a directionwithin the outer angle between the average optic axes (or alternativelythe output directors) of the two regions of the patterned retarder. In apossible further embodiment the patterned and/or non-patterned retardermay be constituted by a switchable liquid crystal device.

EXAMPLE 1

A possible method of fabrication which might be used to produce theembodiment of FIG. 2A will now be described, by way of example. In thismethod the layers 2 and 11 are fabricated using reactive mesogen (RM)materials, as these materials have a high birefringence (Δn−0.15) inrelatively thin retardance layers, as well as being capable of beingpolymerised, for example by exposure to ultraviolet light. Theparticular fabrication method may use a solution of diacrylate RM 257(supplied by Merck Limited, Poole, UK) for producing retarders designedto act as half wave and quarter wave plates at 500 nm.

An alignment layer 31 is first formed on a substrate by spin coating ofa linearly photopolymerisable material such as is described, forexample, in “Surface induced parallel alignment of liquid crystals bylinearly polymerised photopolymers”, Schadt et al., Japanese Journal ofApplied Physics, vol. 31 (1992), page 2155, as shown at a in FIG. 7A.The alignment layer 31 is then exposed to radiation of a first linearpolarisation through a mask 32 so as to photopolymerise the layer 31 inone alignment direction to form the regions A, as shown at b in FIG. 7A.The unexposed regions of the layer 31 are then exposed to radiationhaving a different linear polarisation through a mask 33 so as tophotopolymerise the layer 31 in a different alignment direction to formthe regions B, as shown at c in FIG. 7A. Thus alternate regions A and Bof the alignment layer 31 provide different alignment directions, forexample differing by 45°, which may be used to align different regionsof a subsequently applied RM layer. The alignment layer 31 is thencovered by a retarder layer 34 by spin coating of RM 257 solution, asshown at d in FIG. 7A, and the retarder layer 34 is fixed or polymerisedby exposure to ultraviolet radiation so as to form a patterned uniformretarder 35, as shown at a in FIG. 7A, having regions which take up thealignment of the regions A and B and which correspond to the regions 3and 4 of the patterned layer 2 of FIG. 2A. The non-patterned uniformretarder 15 may then be formed by spin coating of a further layer (notshown in FIG. 7A) of RM 257 solution which is fixed or polymerised byexposure to ultraviolet radiation.

EXAMPLE 2

An alternative method of fabrication which might be used to produce theembodiment of FIG. 2A will now be described. This method will bedescribed only to the extent to which it differs from the method alreadydescribed with reference to FIG. 7A.

In the alternative method the alignment layer 31 is firstunidirectionally rubbed throughout its free surface. For instance, thelayer 31 may be rubbed three times with a rubbing cloth on a rollerrotating at 3000 rpm. After such rubbing, masking of the alignment layer31 is performed using standard photolithographic techniques, that is byspin-coating a layer of positive photoresist on top of the layer 31,soft-baking the photoresist, exposing the photoresist to ultravioletlight through a mask and developing the photoresist to remove thephotoresist from the regions of the layer 31 which are to form theregions B.

The photoresist layer is then hard-baked prior to a subsequent rubbingstep being performed with the second rubbing direction being at apredetermined angle, for example at 45°, to the first rubbing direction.In practice the second rubbing direction may be slightly offset so as tocompensate for the residual effect of the first rubbing. Finally thephotoresist is removed to leave the finished substrate and alignmentlayer to which the retarder layer 34, as shown at d in FIG. 7A, isapplied by spin-coating prior to being fixed or polymerised by exposureto ultraviolet radiation so as to form the patterned uniform retarder 35as shown at e in FIG. 7A.

Such a method of fabrication is disclosed in more detail in BritishPatent Application No. 9804501.6.

The substrate 30 is selected so as to minimise any birefringence whichwould otherwise affect the performance of the device, for example byreducing contrast ratio or degrading the chromatic performance of thedevice. The substrate may, for example, be a suitable float glass ofappropriate flatness. Furthermore the wavelength characteristic of theretarder can be designed by using the Poincare sphere technique asdescribed, for example, in the S. Pancharatnam references mentionedabove, or by use of the well known Jones matrix technique.

The embodiment of FIG. 3 may be produced in a broadly similar mannerexcept that the layer 16 is formed as a uniform half wave plate. FIG. 6Ais a graph showing the transmission of the output light in use of theembodiment of FIG. 3 as a function of the wavelength of the incidentlight. In order to produce this graph the linearly polarised incidentlight 14 was directed onto the device described with the reference toFIG. 3, and an analyser was orientated at 45° to the xz plane and thetransmission of light through the regions 4 measured. The broad bandachromatic response was seen by rotating the analyser to −45°. A similarresponse can be obtained for transmission through the regions 3.

FIG. 7B diagrammatically shows a liquid crystal display (LCD) formed bya display panel 39 including a polariser 40 and an analyser 42 disposedon opposite sides of a liquid crystal layer 41 with alignment layers 43,44 and electrodes 45, 46 being provided on opposite sides of the liquidcrystal layer 41 in known manner. The liquid crystal layer 41 includespixels A and B at the crossing points of the electrodes 45, 46 which maybe addressed by data and scan signals applied to the electrodes 45, 46in known manner. Furthermore a patterned uniform retarder 47 and anon-patterned uniform retarder 48 are applied to the analyser 42. Thepatterned retarder 47 consists of a patterned half wave plate havingregions 49 aligned with the pixels A and regions 50 aligned with thepixels B. The regions 49 and 50 have their optic axes arranged at +22.5°and −22.5° to the output polarisation state of the LCD panel 39, and thenon-patterned retarder 48 is constituted by a uniform half wave platehaving its optic axis at 90° to the output polarisation state of the LCDpanel 39.

Such a LCD may be viewed by the two eyes 53 and 54 of a viewer throughspectacles having polarisers 51 and 52 with transmission axes arrangedat −45° and +45° respectively to the output polarisation state of theLCD panel 39 in order to observe a stereoscopic image made up of imagecomponents transmitted by the pixels A and B respectively. Alternativelyone set of viewers may observe the LCD through polarisers withtransmission axes at −45°, whilst another set of viewers observes theLCD through polarisers with transmission axes at +45°. As a result thetwo sets of viewers receive different information from the LCD, allowingtwo channels of information, such as two television channels or acomputer game and a television channel or two documents, to beconcurrently viewed using a single LCD panel.

FIG. 8 diagrammatically shows a diffractive optical system 60 includingtwo sets of interleaved electrodes 61, 62 provided on a substrate 70 anddisposed on the opposite side of a surface stabilised ferroelectricliquid crystal (SSFLC) layer 71 to an electrode 63 provided on asubstrate 73. The sets of electrodes 61 and 62 are at differentpotentials to one another, for example at +V₀ and −V₀, and to theelectrode 63 which is at 0 volts, for example. The SSFLC layer 72 isarranged such that, under these conditions, the optic axes of theregions A and B adjacent to the electrode sets 61 and 62 arerespectively at +22.5 and −22.5% to an arbitrary reference axis. Alsoprovided on the substrate 70 are a quarter wave retarder 64 having itsoptic axis at 90 degrees to the arbitrary axis and a mirror 65. Underthese conditions unpolarised light 66 incident on the system 60 isdiffracted by the regions A and B and the quarter wave retarder 64 intoreflected non-zero orders. The diffraction has good achromaticitycompared with the case in which a quarter wave retarder is providedwhich is orientated at 0 degrees to the arbitrary reference axis. If theelectrode sets 61 and 62 are then both set to the same potential +V₀with respect to the electrode 63 at 0 volts, the optic axes of theregions A and B of the SSFLC layer 72 become parallel, and nodiffraction of the unpolarised light 66 into reflected non-zero ordersoccurs. Thus, by switching of the electrodes 62, the patterning of thelayer 72 can be turned on and off to provide an enhanced switchableachromatic diffractive optical system.

INDUSTRIAL APPLICABILITY

As described above, the invention makes possible the advantage ofproviding an optical retardation device having improved achromaticitywhich enables generation of broad band orthogonal optical modes andwhich permits a broad bandwidth response to be obtained with improvementin the quality and/or ease of fabrication as compared with knownbroadband optical retardation devices. The broadband optical retardationdevices may be used for polarisation encoding of display information orin diffractive optical systems, for example.

What is claimed is:
 1. A broadband optical retardation device forreceiving light consisting of a broad range of wavelengths, the deviceincluding patterned optical retardation means including a first regionhaving a first optic axis at an orientation a between 0° and +90° to areference plane and a second region in a same plane as the first regionand having a second optic axis at an orientation b between 0° and −90°to the reference plane, and non-patterned optical retardation meanshaving an optic axis at a defined orientation c, greater than theorientation a and less than the orientation 180°+b, to the referenceplane in order to increase the achromaticity of the light polarised bythe combination of the patterned optical retardation means and thenon-patterned optical retardation means as compared with light polarisedby the patterned optical retardation means alone, and wherein theorientations a and b of the first and second optic axes of the patternedoptical retardation means are substantially equal and opposite relativeto the reference plane, and the orientation c of the optic axis of thenon-patterned retardation means is substantially perpendicular to thereference plane.
 2. A device according to claim 1, wherein the first andsecond regions of the patterned optical retardation means are such as topolarise input light linearly polarised along the reference plane suchthat, after passing through the non-patterned optical retardation means,the polarisation of the light which has passed through the first regionis orthogonal to the polarisation of the light which has passed throughthe second region.
 3. A device according to claim 1, wherein of theorientations a and b of the first and second optic axes of the patternedoptical retardation means are in the ranges of +10° to +75° and −10° to−75° respectively relative to the reference plane.
 4. A device accordingto claim 3, wherein the orientations a and b of the first and secondoptic axes of the patterned optical retardation means are in the rangesof +10° to +35° and −10° to −35° respectively relative to the referenceplane.
 5. A device according to claim 4, wherein the orientations a andb of the first and second optic axes of the patterned opticalretardation means are about +22.5° and −22.5° respectively relative tothe reference plane.
 6. A device according to claim 1, wherein theorientations a and b of the first and second optic axes of the patternedoptical retardation means and the orientation c of the optic axis of thenon-patterned optical retardation means substantially satisfy therelationship c=a+45° and b=c−90°.
 7. A device according to claim 6,wherein the orientations a and b are about +22.5° and −22.5°respectively and the orientation c is about +67.5° relative to thereference plane.
 8. A device according to claim 1, wherein the patternedoptical retardation means comprises patterned uniform layer having anoptic axis which varies between the first and second regions along oneor more directions x and y parallel to the layer, but which does notvary substantially through the thickness of the layer.
 9. A deviceaccording to claim 1, wherein the patterned optical retardation meanscomprises patterned twisted retardation layer having an optic axis whichvaries between the first and second regions along one or more directionsx and y parallel to the layer and also through the thickness of thelayer.
 10. A device according to claim 1, wherein the non-patternedoptical retardation means comprises a uniform retardation layer whoseoptic axis has an orientation which does not vary substantially throughthe thickness of the layer.
 11. A device according to claim 1, whereinthe non-patterned optical retardation means comprises a twistedretardation layer whose optic axis has an orientation which variesthrough the thickness of the layer.
 12. A device according to claim 1,wherein the patterned optical retardation means is a half wave plate.13. A device according to claim 1, wherein the non-patterned opticalretardation means is a half wave plate.
 14. A device according to claim1, wherein the non-patterned optical retardation means is a quarter waveplate.
 15. A device according to claim 1, wherein the patterned opticalretardation means incorporates third regions having a third optic axisat an orientation between a and b.
 16. A device according to claim 15,wherein the orientations a and b of the first and second optic axis ofthe patterned optical retardation means are about +22.5° and −22.5° tothe reference plane, the orientation of the third optic axis is at about0° to the reference plane, and the orientation c of the optic axis ofthe non-patterned optical retardation means is about 90° to thereference plane.
 17. A device according to claim 1, wherein thepatterned optical retardation means comprises a suitable liquid crystaldevice.
 18. A device according to claim 1, wherein the non-patternedoptical retardation means comprises a suitable liquid crystal device.19. An optical display system incorporating an optical retardationdevice according to claim
 1. 20. A diffractive optical systemincorporating an optical retardation device according to claim
 1. 21. Adevice according to claim 1, wherein the non-patterned opticalretardation means is provided in optical series with the patternedoptical retardation means.
 22. An optical retardation device comprisingpatterned optical retardation means comprising a first region having afirst optic axis configuration and a second region in a same plane asthe first region and having a second optic axis configuration, andnon-patterned optical retardation means having a further optic axisconfiguration, at least one of the optic axis configurations being atwisted optic axis configuration whose optic axis has an orientationwhich varies through the thickness of a layer, whereby the achromaticityof the light polarised by the combination of the patterned opticalretardation means and the non-patterned optical retardation means isincreased as compared with light polarised by the patterned opticalretardation means alone, and wherein the orientations a and b of thefirst and second optic axes of the patterned optical retardation meansare substantially equal and opposite relative to the reference plane,and the orientation c of the optic axis of the non-patterned retardationmeans is substantially perpendicular to the reference plane.
 23. Adevice according to claim 22, wherein the first optic axis configurationhas an average optic axis orientation a between 0° and +90° to areference plane, the second optic axis configuration has an averageoptic axis orientation b between 0° and −90° to the reference plane, andthe further optic axis configuration has an average optic axisorientation c, greater than the orientation a and less than theorientation 180°+b, to the reference plane.
 24. A device according toclaim 22, wherein the first optic axis configuration has an outputdirector orientation a between 0° and +90° to a reference plane, thesecond optic axis configuration has an output director orientation bbetween 0° and −90° to the reference plane, and the further optic axisconfiguration has an output director orientation c, greater than theorientation a and less than the orientation 180°+b, to the referenceplane.
 25. A device according to claim 22, wherein the first and secondregions of the patterned optical retardation means are such as topolarise input light linearly polarised along a reference plane suchthat, after passing through the non-patterned optical retardation means,the polarisation of the light which has passed through the first regionis orthogonal to the polarisation of the light which as passed throughthe second region.
 26. A device according to claim 22, wherein thefurther optic axis configuration is uniform having an optic axisorientation which does not vary substantially through the thickness of alayer.
 27. A device according to claim 22, wherein the first and secondoptic axis configurations are uniform having optic axis orientationswhich do not vary substantially through the thickness of a layer.
 28. Adevice according to claim 22, wherein the non-patterned opticalretardation means is provided in optical series with the patternedoptical retardation means.